This invention relates generally to fusing systems utilized for pressure fixing toners at elevated temperatures in electrostatic copying devices and more particularly to an improved wick member for applying release fluid to fuser members in such fusing systems.
In the process of xerography a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic particles, commonly referred to as toner. The visual toner image can be fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto. Toners are well known in the art and may be of various types.
In order to affix or fuse electroscopic toner material onto a support surface permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent into the fibers or pores of support members of otherwise upon the surface thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy for fixing toner images onto a support member is old and well known.
Several approaches to thermal fusing of electroscopic toner images onto a support have been described in the prior art and include providing the concomitant application of heat and pressure as by a roll pair maintained in pressure contact, a flat or curved plate member in pressure contact with a roll, a belt member in pressure contact with a roll, and the like. Heat may be applied by heating one or both of the rolls, plate members or belt members. The fusing of the toner takes place when the proper combination of heat, pressure and contact time are provided, the balancing of these parameters being well known in the art and varying according to various factors which must be independently determined for each particular situation.
During operation of a fusing system of the type where there is a thermal fusing of electroscopic toner images onto a support in which at least one fuser member, such as a roll, plate or belt, is heated, the support member to which the toner images are electrostatically adhered, is moved through the nip formed between the members with the toner image pressure contacting the fuser roll thereby to effect heating of the toner images within the nip. By controlling the heat transfer to the toner, virtually no offset of the toner particles from the copy sheet to the fuser member is experienced under normal conditions. This is because the heat applied to the surface of the fuser member is insufficient to raise the temperature of the surface of the member above the "hot offset" temperature of the toner at which temperature the toner particles in the image areas of the toner liquify and cause a splitting in the molten toner resulting in "hot offset". Splitting occurs when the cohesive forces holding the viscous toner mass together is less than the adhesive forces tending to offset it to a contacting surface such as a fuser roll, fuser belt, or fuser plate.
Occasionally, however, toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof (i.e. "cold offsetting"); by imperfection in the properties of the surface of the roll; by the toner particles insufficiently adhering to the copy sheet; by the electrostatic forces which normally hold them there; or by the reactivity of the toner material itself in those cases where the toner is of a reactive nature. In such a case, toner particles may be transferred to the surface of the fuser member with subsequent transfer to the backup member which provides pressure contact, during periods of time when no copy paper is in the nip.
One arrangement for minimizing the foregoing problems, particularly that which is commonly referred to as "offsetting", has been to provide a fuser member with an outer surface or covering of polytetrafluoroethylene, known by the tradename Teflon, to which a release agent such as silicone oil, is applied. More recently, bare metal fuser members have been introduced for fusing or fixing the electroscopic toner materials to various surfaces. Various fluid polymer release materials which oxidize or which contain functional groups, can be utilized to prevent "offsetting". Exemplary of such systems are the disclosures in U.S. Pat. No. 3,937,637 and U.S. Pat. No. 3,918,804. Other release agents for bare metal fuser rolls are described in Belgium Pat. No. 831,662.
In the foregoing exemplary fusing systems the release agent or release fluid may be applied to the fuser member by means of a wick as described in U.S. Pat. No. 3,718,116, U.S. Pat. No. 3,831,553 and U.S. Pat. No. 3,841,827. The wick is generally used to dispense silicone oil, functional siloxane fluids, mineral oil, and many other release fluids upon the external surface to the fuser member in the form of a pad overlying and in contact with the fuser member which is heated during operation.
As described in the foregoing patents, the wick assembly generally includes two different layers. A first layer in contact with the surface of the fuser member meters precise amounts of release fluid thereon, while a second layer in contact with the first layer has high release fluid retention capabilities for supplying the first layer with the fluid. In a preferred embodiment, the wick comprises a layer of Teflon which contacts the surface of a fuser roll, and a second layer of Nomex which has its underside in contact with an applicator roll, the release fluid in a sump or some other fluid supply device or means. Teflon and Nomex are trademarks of E. I. du Pont de Nemours & Company of Wilmington, Delaware.
The prior art wick assemblies comprising two layers, such as Teflon and Nomex, are joined by stitching, clamping or cementing the layers together. These methods of fabricating the two layers have many disadvantages. First, these conventional wicks are relatively inefficient in transporting fluid, and there is relatively short wick like due to separation of significant portions of the two layers. Secondly, because of the tendency for layers of the prior art wicks to separate from each other, and because there is only surface to surface contact of the two layers with each other, there is low fluid transfer or through put from one layer to the other layer.
In the prior art methods of fabricating wick materials, there is also a tendency for the fibers in the layers to be loosely held, thereby causing accumulations of fuzz(lint) and/or fibers in various machine parts. These can cause serious problems in copy quality, especially when the fibers accumulate in critical such as in metering areas resulting in non-uniform metering and oil streaks on copies. Toner build-up on these fibers is transferred back to subsequent copies as toner offset. In fact, the second layer is frequently flame treated prior to use in a fuser assembly to burn all loose fibers.